Multiplex control system having enhanced integrity

ABSTRACT

A serial data communications system, particularly for use in an automotive environment is disclosed in which bidirectional serial data communication is effected between a central control station and one or more remote stations with high integrity. The central control station includes a central controller and at least two multiplex controllers. A serial signal bus is arranged as a loop connected at its opposite ends to respective ones of the two multiplex controllers. A remote multiplex controller, located at each of the remote stations, includes input/output signal ports for input and output, respectively of signals respectively from and to remote input devices and remote load devices. Each remote multiplex controller is parallel &#34;T&#34; connected to the serial bus intermediate its end terminals, has a respective address, and is responsive to a serial communication containing its respective address from the central control station for providing a response to the communication. The central controller selects one of the multiplex controllers to be the master in accordance with a preselected bias, and selects the other of the multiplex controllers to be the master when data communication of acceptable integrity with an addressed one of the remote controllers by the designated master multiplex controller is unattainable.

DESCRIPTION

1. Technical Field

This invention relates to a serial data communications system and moreparticularly to a system for providing bidirectional serial datacommunication between a central control station and one or more remotestations. More particularly still, the invention relates to theprovision of enhanced integrity in such communications systems.

2. Background Art

Serial data communication between a central control station and variousremote stations using multiplexing techniques is well known,particularly in the communications industry. More recently, suchmultiplexing techniques have been applied in the automotive and otherallied industries for effecting control at plural remote locations via acentral controller typically employing a microprocessor. In many suchapplications, the data communications links are in a relatively"friendly" environment and/or their continued integrity is assured.Further, even though such systems may be exposed to environments inwhich electromagnetically-generated signal interference may sometimesexist, there may be no critical need for complete integrity of the datatransmission process.

On the other hand, certain environments such as that of an automobileand possibly other vehicles may be particularly hostile to the continuedphysical integrity of the data communication paths or conductors.Further, such an environment may give rise to considerableelectromagnetic interference, and the performance of a control functionrequested or commanded via a communications link in that environment maybe jeopardized.

Examples of communications systems employing serial multiplex data linesextending from a central controller to remote controllers in automobilesand other vehicles include U.S. Pat. Nos. 3,585,595; 3,737,862;4,085,403; 4,164,730; and 4,298,930. The first two mentioned patents aeintended for use aboard aircraft and provide two separate data pathsthrough two separate data loops for sequentially determining controlswitch inputs in sequence at respective coders and then sequentiallyimplementing the requested action via series connected decoders in theother data loop. Those systems rely upon the integrity of each and everyserial coder and/or decoder in the respective data loops for passingsignals to the next successive code or decoder. To provide for linebreaks or coder or decoder short circuits, those patents provide one ortwo column timers connected to the control loops and themselvescontrolled in a manner which permits the sequential addressing of thedata loops in either of opposite directions. Those systems, however, arelimited by factors such as the need to first address the coders insequence and to then readdress the decoders in the same sequence. Such asystem has little or no flexibility in terms of the addressability ofthe various remote stations. Moreover, those systems do not appear toidentify the location at which a line break might have occurred.

The other three above-cited patents, i.e., above 4,000,000, illustrateautomotive communications systems which appear to rely upon a ring busclosed upon itself and controlled at a "T" junction by a central stationfor delivering signals in both directions about the ring to receiversconnected to the ring bus by respective "T" connections. With suchconfiguration, communication in both directions is provided. However,the quality or integrity of those communications may not be acceptabledue to signal attenuation or degradation or electromagneticinterference, and those systems do not appear to provide for recognizingand compensating for such shortcomings. Moreover, those systems appearto require a feedback or "acknowledge" line in their serial bus inaddition to the conventional power, clock and control lines. Thisrequirement of an extra line in the serial bus may be consideredundesirable, depending upon its length, cost, weight and susceptibilityto electromagnetic interference.

DISCLOSURE OF INVENTION

One object of the present invention is to provide a serial datacommunication system having enhanced integrity. Another object of thepresent invention is the provision of a serial data communicationssystem suitable for use in an automotive environment for providingreliable communication between a central control station and one or moreremote stations at which switch inputs and/or output loads are located.It is yet another object of the present invention to provide acommunications system in which the controllers at the remote stationspossess considerable "intelligence" for aiding in the provision ofenhanced communications integrity.

According to the present invention, there is provided a serial datacommunications system in which bidirectional serial data communicationis effected between a central control station and one or more remotestations with high integrity. The central control station includes acentral controller, which may be a microprocessor with associatedmemory, and at least two multiplex controllers. A serial signal bus isarranged as a loop connected at its opposite ends to respective ones ofthe two multiplex controllers. A remote multiplex controller, located ateach of the remote stations, includes input/output signal ports forinput and output, respectively of signals respectively from and toremote input devices and remote load devices. Each remote multiplexcontroller is parallel "T" connected to the serial bus intermediate itsend terminals, has a respective address, and is responsive to a serialcommunication containing its respective address from the central controlstation for providing a response to the communication. The remotecontroller's response includes the input of signals from remote inputdevices and/or the output of signals to remote load devices and inaddition, includes a data reply to the serial bus via the "T" connectiontherewith. The central controller, which may conveniently be amicroprocessor, is connected for communication with each of themultiplex controllers via parallel buses. Each of the multiplexcontrollers is selectably responsive to instruction by the centralcontroller for effecting serial bidirectional data comunicationtransactions as a master with addressed ones of the remote controllers.Moreover, the central controller includes sufficient signal processingcapability for selecting one of the multiplex controllers to be themaster in accordance with a preselected bias and for selecting the otherof the multiplex controllers to be the master when data communication ofacceptable integrity with an addressed one of the remote controllers bythe designated master multiplex controller is unattainable.

Each of the central station multiplex controllers is of substantiallythe same architecture and includes a respective control register foreffecting operation of the controller as either a master or a monitor. Amonitor multiplex controller is able only to receive data on the serialbus as transmitted from the master controller and as sent in the repliesfrom the respective remote controllers responding to the mastercontroller. The central processor sets a particular logic state in aparticular stage of the multiplex controller's control register forestablishing it as either a master or a monitor, and the system includesonly one master at anytime.

The system includes the capability of comparing a portion of the datareply from a remote controller with a portion of the data transmissionfrom a master controller to provide signals indicative of whether or notcommunication of acceptable integrity is attainable with a particularaddressed remote controller. Both the master and the monitor multiplexcontrollers also include logic for determining whether or not anattempted communication from the master to an addressed remotecontroller is of acceptable integrity. Typically an error check byte maybe established by the master on its outgoing transmission and a separateerror check byte may be established by the remote controller on itsreply transmission. The monitor multiplex controller may, because itpresumably is monitoring both sides of the transaction, similarlycompute an error check byte for purposes of comparison. In both themaster and monitor controllers, a determination of comparability ornoncomparability is stored as a status bit in a respective statusregister. The system additionally includes capability for comparingaddresses transmitted by the master with addresses from the respondingremote muxes to determine the integrity of the attempted communication.

The program of the central processor is operative upon initial start-upof the system to address the remote controllers in a predeterminedpolling sequence. The initial polling of the remote controllers servesto determine the selection thereafter of a particular multiplexcontroller for use as a master to address a particular remotemultiplexer. The multiplex controller preselected to be the master priorthe initial polling is preferred in the selection determination for eachparticular addressed remote multiplexer with which communication issatisfactory during the initial polling sequence. Otherwise, themultiplex controller preselected to act as monitor is selected to actinstead as master for communication with the particular remotemultiplexer.

The processor's program for selecting the direction of datacommunications about the serial loop by configuring a particularmultiplex controller either as a master or as a monitor is effected as aresult of several of the aforementioned communications-integritycriteria failing to be satisfied. Moreover, before the decision isfinally made to configure a particular multiplex controller as a masteror a monitor for addressing a particular remote multiplexer, severalattempts will be made to complete the communication transaction usingthe original configuration.

The serial bus consists of four conductors including one for serialmultiplex data, one for multiplex clock signals, one for a first logicpower level, such as a logic 1, and a final conductor for the otherlogic power level, for instance logic 0 at ground. The multiplexcontrollers each include logic for generating an interrupt signalextendible to the central processor; however, only that controllerconfigured as a master is enabled to issue the interrupt signal tothereby initiate a data request by the microprocessor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an architectural block diagram of a communication system inaccordance with the present invention;

FIG. 2 is a generalized schematic block diagram of the multiplexcomputer comprising part of the embodiment of FIG. 1;

FIGS. 3A and 3B in combination are a more detailed schematic blockdiagram of the multiplex computer of FIG. 2, with all of the circuitryof FIG. 3A being powered in a switch-controlled manner and all of thecircuitry of FIG. 3B being continuously powered;

FIG. 4 is a schematic block diagram of a Master/Monitor Mux Controllerused in the multiplex computer;

FIG. 5 is a series or waveform illustrations A-L of particular signalsassociated with the Controller illustrated in FIG. 4;

FIG. 6 is a schematic block diagram of a Remote Mux Controller used inthe system illustrated in FIG. 1;

FIG. 7 is a schematic diagram of debounce circuitry and latch circuitryassociated with the Remote Controller;

FIG. 8 is a series of waveform illustrations used in the description ofthe embodiment of FIG. 7;

FIG. 9 is a schematic diagram of address input and signal outputcircuitry used with integrated circuit devices employed in theembodiment of FIG. 7;

FIG. 10 is a series of waveform illustrations used in the description ofthe embodiment of FIG. 9;

FIG. 11 is a flow diagram of the decision and control routine associatedwith configuring the Multiplex Controllers as Master and Monitor forcommunications integrity for the system;

FIG. 12 is a flow diagram of the decision and control routine associatedwith an evaluation of the integrity of the serial communications;

FIG. 13 is a flow diagram of the decision and control routine associatedwith the "sleep" mode of system operation.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 is an architectural block diagram of the multiplex communicationsystem 10 for the body electrical functions of a vehicle andincorporating the present invention. In the interest of brevity, severalabbreviations or shorthand expressions will be used hereinafter in placeof the full descriptive name or function of an element, for instance,the word "multiplex" will often be expressed as "Mux" and a remotemultiplexer will be referred to as a "Remux". Further still, thedesignations for various signals appearing on various conductors or atvarious ports in the system will be represented by descriptiveabbreviations. Still further, the logic employed in the illustratedembodiment uses, in many instances, the "active-low" state of a signalfor effecting some result. Although that "active-low" state isrepresented in the Drawings by a line or "bar" over the signalexpression, that same "active-low" state is represented in the text bya * adjacent to the signal representation because of printinglimitations. The multiplex communication system 10 employs a Muxcomputer 12 located at a central station within an automotive vehiclefor providing control to and interacting with one or more remotemultiplex (Remux) controllers 14 positioned at various remote locationsabout the vehicle. Communication between the Mux computer 12 and theRemuxes 14 is afforded via a four wire bus 15 which includes a firstwire 16 for carrying bidirectional, serial, time division multiplexeddata, a second wire 17 for conveying the serial multiplex clock(MUXCLK), a third wire 18 for extending a +5 volt DC supply voltage tothe Mux computer 12 and the Remuxes 14 and a fourth wire 19 which servesas a signal ground (GND) for the multiplex system 10. The five voltsupply voltage and the ground may be supplied by and referenced to theconventional 12 volt battery (not shown) of an automotive vehicle via a5 volt regulator 20.

Although the multiplex communications bus 15 between the Mux computer 12and the various Remux controllers 14 might in some applications be openended with the Mux computer 12 being located at one end and the variousRemux controllers 14 tapping in parallel "T" connection to theconductors 16-19 along the length of the bus, in accordance with anaspect of the present invention the bus 15 is formed as a loop which isterminated at its opposite ends or terminals by differing portions ofthe Mux computer 12 to provide increased integrity and security to themultiplex communication system 10 as will be hereinafter described.Provision of a communication bus 15 configured as a loop controlled ateach end by the Mux computer 12 permits detection of various anomolieswhich may occur in the transfer of information, such detection being ofa nature to identify one or more breaks in the bus 15 and to furtherprovide for maintaining transmission integrity in the event of such linebreak.

The Mux computer 12 located at the central station includes a standardmicroprocessor 22 operatively connected with a master Mux controller 24and a monitor Mux controller 24'. The master Mux controller 24 (MasterMux) and the monitor Mux controller 24' (Monitor Mux) are each formed ofcustom LSI CMOS gate array circuitry and are identical in constructionbut differ somewhat in operation as a function of time and control mode.One end of the loop of multiplex bus 15 is connected to Master Mux 24and the other end is connected to Monitor Mux 24'. The microprocessor 22in the preferred embodiment is a 4 MHz Z80, such as the Mostek 3880,employing NMOS circuitry, but it will be understood that othermicroprocessors are similarly applicable. Memory 25 is also provided inconjunction with microprocessor 22 in the Mux computer 12, and typicallyincludes a 1K CMOS random access memory (RAM) 26 and a 4K CMOSprogrammable read only memory (EPROM) 27 shown in FIG. 2. Mux computer12 also includes oscillator and counter/timer circuitry, generallyrepresented by the function block 28, for generating the system timingsignals and for providing a "sleep mode" of operation to be describedhereinafter in greater detail. Operative interconnection between themicroprocessor 22, memory 25, oscillator and counter/timer 28 and theMaster and Monitor Muxes 24, 24'is afforded by various control linesshown later in greater detail as well as by a data bus and an addressbus, so designated, within the functional illustration of Mux computer12.

All of the Remuxes 14 connected to the multiplex bus 15 are of similarconstruction, each being an LSI gate array employing CMOS logicelements. Each Remux controller 14 is provided with significant logiccapacity for "intellectual" interaction with the Mux computer 12, andtypically includes provision for 16 inputs from various vehicle switchesand 16 outputs to various vehicle loads, a typical switch input beingthat for on-off control of headlights and a typical output being acontrol signal for turning on or turning off the headlights. It will beappreciated that the switch inputs may derive from the need to controlnumerous diverse loads and functions and similarly, that the outputsignals will serve to control numerous diverse types of loads. Thearchitecture of each Remux 14 is such that it is divided into halves,each half having eight inputs and eight outputs and having a separateaddress. More specifically, one side of a Remux 14 is designated the Aside and is provided with an even numbered address and the other side isdesignated the B side and is given an odd number address which isnumerically 1 greater than the A side address. The outputs from Remuxes14 to the various vehicle loads typically provide low voltage controlsignals to various control elements or buffers 30 which in returnrespond by connecting or disconnecting the vehicle 12 volt supply to theload being controlled.

The multiplex communication system 10 employs a communications protocolillustrated in FIG. 5B for use in the data communications between theMux computer 12 and the Remuxes 14. This data protocol is intended toenhance the integrity of the communications system through an efficientdetection of communication errors and/or anomolies. A thoroughdescription of this data protocol is contained in U.S. application Ser.No. 469,591 for Vehicle Multiplex System Having Protocol/Format forSecure Communication Transactions filed Feb. 24, 1983 by William Floydand assigned to the same assignee as the present application, and isincorporated herein by reference. Briefly, each communicationtransaction on the Mux data line 16 of multiplex bus 15 includes sevencharacters or bytes of eight bits each, the first byte being a syncbyte, the following three bytes comprising a command message from themaster controller 24 consisting of an address byte, a command byte and aCRC error detect byte and the final three bytes comprising a replymessage from a Remux 14 consisting of an address byte, a response byteand a CRC error detect byte. Although the system 10 as presentlyconfigured has the potential for controlling as many as 128 Remuxes 14,each having two separate addresses, it will be understood that usuallyfar fewer Remuxes 14 are actually required, only two being illustratedin the FIG. 1 embodiment.

Referring briefly to FIGS. 2, 3A and 3B, the sysem clock (SYSCLK) forcontrolling the timing of the microprocessor 22 and various otherelements of the Mux computer 12 is typically of 2.5 MHz and is providedby an RC oscillator 28 of conventional design. Further, a similar typeof RC oscillator provides a 50 KHz source 30 which is divided by two inthe initial stage of a multistage binary counter 32 to provide a 25 KHzclock signal which is used as the multiplex clock (MUXCLK). Inasmuch asthe serial Mux data appearing on multiplex bus 15 is clocked at a 25 KHzrate, the period of each bit will typically be 0.04 ms and the period ofan eight-bit byte is 0.32 ms. Interactions between the microprocessor22, its program stored in EPROM or PROM 27, the data stored in RAM 26,and the Master and Monitor Muxes 24, 24' is conducted at a ratedetermined by SYSCLK. Interaction between the microprocessor 22 and thememory elements 26 and 27 is determined by conventional decode controllogic 34 which additionally includes timer decode logic for providingcertain control signals to timer circuitry and power control switchcircuitry generally represented by the blocks designated 31 and 35,respectively in FIG. 2 to be described hereinafter in greater detail.The processor 22 is interrupt driven, a nonmaskable interrupt (NMI)*being provided by the timer 31 at approximately 20 ms intervals. Thehardware timer 31 includes two, eight-stage, divide-by-256 counters, 32and 33, respectively of FIG. 3B. A timing signal occurring at 20 msintervals from the timer 31 is applied to pulse generating circuitry,generally designated 36 in FIG. 2 for providing the NMI* signal.Additionally, upon initial powering-up of the multiplex system 10, as byconnecting of the vehicle's battery cable, a conventional power up resetsignal PUR₁ is generated by conventional circuitry represented by block37. This PUR₁ signal may also be applied to the pulse generatingcircuitry 36.

As is evident in FIGS. 2, 3A, and 3B, processor 22 utilizes eightbidirectional data lines D₀ -D₇ for the parallel output and input ofdata to the memory elements 26, 27 and the Master and Monitor Muxes 24,24'. The processor also includes 16 address lines A₀ -A₁₅ for providingaddressing signals to the memories 26, 27 and Master and Monitor Muxes24, 24', as well as to the decode control logic 24. The data bus betweenprocessor 22 and the Master and Monitor multiplexers 24, 24' isdesignated 40 and the corresponding address bus between those elementsis designated 42 and consists of A₀ -A₉. Five further signal lines areprovided at the processor 22 and at the Master and Monitor multiplexers24, 24', and are designated RD*, WR* , IORQ*, Ml*, and INT*. The RD*signal is issued by processor 22 when it wishes to read data from memory25 or an I/O device such as the multiplexers, 24, 24'. Depending onwhich is addressed by processor 22, one or the other of the multiplexers24, 24' is issued the RD* signal to gate data onto the data bus 40 frommultiplexers 24, 24'. The WR* signal provided by processor 22 indicatesthat the data bus 40 holds valid data to be stored in the addressedmemory 26, 27 or I/O device multiplexers 24, 24'. The IORQ* signalindicates that the address bus 42 contains a valid I/O address for anI/O read or write operation. This signal is also generated with an Ml*signal when an interrupt is being acknowledged to indicate that aninterrupt response vector can be placed on data bus 40. The Ml* signalindicates that the current processor cycle is the OP-Code Fetch cycle ofan instruction execution, and it also occurs with IORQ*, as mentioned,to indicate an interrupt acknowledge cycle. The INT* signal is onegenerated by a multiplexer operating in its Master Mode, as MasterMultiplexer 24, and is directed to processor 22 when an interrupt isbeing requested. This request is honored by processor 22 at the end ofthe current instruction being executed by the processor.

The processor 22 issues a further control signal designated MREQ* whichis extended to the RAM and EPROM decoding circuitry and to the NMI*pulse generator for selecting either ROM or RAM when the address holds avalid address for a Memory Read or Memory Write operation, and enablingthe NMI* generator to provide a pulse for the NMI* input.

Referring to FIG. 3B, it will be noted that Master and MonitorMultiplexers 24 and 24' each include address input lines ADDCMP 1-7 forfixing or hardwiring their respective addresses. With respect to MasterMonitor 24 it will be noted that all of the ADDCMP 1-7 inputs are tiedto ground (logic 0) except for ADDCMP 4 which is at +5 volts (logic 1).A similar situation exists with Monitor Multiplexer 24' except that theinput ADDCMP 4 is a logic 0 and input ADDCMP 5 is a logic 1.Accordingly, the Master and Monitor Muxes 24, 24' are separatelyidentified and identifiable in their respective communications withprocessor 22.

Each of the Master and Monitor Mux modules 24, 24' also includes aninput designated MR which receives a signal designated PUR₂ obtained ina manner to be hereinafter described and which effects a resettinginitialization of the internal control registers and timing of therespective Muxes 24, 24'.

At this juncture it is appropriate to further consider the operatingprotocol of the multiplex system 10 particularly as regards serial datamultiplex transactions between the Mux computer 12 and the Remuxes 14.In accordance with the routine of programmed instructions contained inROM 27, the processor 22 scans the various Remuxes firstly to establishwhat, if any, input switches have been actuated, and secondly to effectthe requisite output control action to the appropriate loads. To effectthis control, the processor 22, which typically provides and receivesoperating addresses and data in a parallel manner, utilizes the MasterMultiplexer 24 to convert the address and command issued to therespective Remuxes 14 into a serialized data format and to reconvert theserialized address and response data issued by the Remuxes 14 into aparallel format for transmission on the parallel data bus 40 toprocessor 22. As mentioned earlier, a typical transaction between theMux computer 12 and a Remux 14 includes, as illustrated in the trace ofFIG. 5B, the issuance of a sync byte followed by an address byte, acommand byte and an error check (CRC) byte in the message sent from theMaster Mux 24 to a particular Remux 14. Subsequently the addressed Remux14 will, or should, reply on Mux data line 16 with an address byte, aresponse byte and an error check (CRC) byte. The address byte sent bythe Master Mux 24 contains the address of a selected half of one of theRemuxes 14. The command byte instructs the addressed Remux to respondwith various input signals which it may have received by the actuationof outside switches and/or to provide output control signals to theoutput load devices connected to that particular half of the Remux. Acyclical redundancy error checking technique utilizes the address andcommand bytes for generating an error check byte which is transmitted tothe addressed Remux. The Remux 14 which responds is presumably thatwhich was addressed by Master Mux 24 and the response is initiated withan address byte which indicates the address of the particular respondingRemux. That byte is followed by a response byte which indicates theresponse taken by the particular Remux to the received command message;typically revealing the status of various input switches and actuationof output loads. In this regard, the status of switch inputs and/oroutput loads is typically determined by sampling latched switch inputsand actuation responses of load outputs. The response byte willtypically also include an indication of whether or not the particularRemux, having done its own error checking of the incoming message fromMaster Mux 24, "agrees" with its error check byte. Finally, the errorcheck byte sent by the Remux 14 will have been calculated using a CRCtechnique using its Address and Response in the reply message. TheMultiplexer does its own error checking on the Remux Reply. Followingeach such transaction there may or may not be a period of bus latency inwhich all "ones" are written while a further instruction is awaited fromprocessor 22.

Referring now to FIG. 4, the Master and Monitor Muxes 24, 24' areconsidered in greater detail. Because both are of identicalarchitecture, they are discussed and illustrated as one in FIG. 4,though they are capable of functioning differently from one another inaccordance with the invention. Accordingly, the discussion will first befrom the standpoint of the Master Mux 24 and subsequently the MonitorMux 24'. Further in FIG. 1 the bus 15 and its MUX DATA line 16 andMUXCLK line 17 are each shown with arrows at both ends, the solidarrowheads depicting the general flow in the presently discussed andillustrated configuration, and the dotted arrowheads depicting thereversed configuration.

When power is initially applied to the Muxes 24 and 24', their initialstates will be that of a monitor. This means they are both in thereceiver or listener mode and are searching for the SYNC byte on theserial Mux data line 16. To convert Mux 24 to the Master status, itseight bit control register 46 must be programmed. This is accomplishedby processor 22 addressing the Mux 24 using the Address Bus 42 andcomparing the address in the Address Compare Logic 48 with the device'saddress established at the inputs ADDCMP 1-7. Assuming further that theA₀ bit of the address sent is a 1, a CNTLCOMP signal is issued to thecontrol register 46 such that when the processor 22 executes a writeoperation, WR, control data will enter register 46 from data bus 40. Theprogramming of the control register 46 for the Master Multiplex functionincludes setting bit number 4 to a 1 to reset the internal logic in theMux module. Once completed, bit number 4 automatically resets itself.Bit number 2 of register 46 is set to a 1 to enable the next byte onData Bus 40 to be written into the Vector Address Register 50. Oncecompleted bit number 2 automatically resets itself. Bits number 0, 1 and3 must be set to 1 for the Mux to operate as a master. Specifically, ifbit number 0 is a 1, the device is a master, otherwise it is a monitor.With bit number 1 as a 1, the Mux clock, MUXCLK, is transmitted on line17 of bus 15. With bit number 3 as a 1, the Interrupt Logic 52 isenabled. Bit numbers 5, 6 and 7 may be programmed to any state sincethey are not used.

Following programming of Control Register 46 such that the device 24will act as a Master Mux, it is again addressed by processor 22 and themultiplexing operation is initiated by writing the appropriate data byte(Address byte) into the Multiplex Register 54. The address comparison atlogic 48 for entering data in Multiplex Register 54 is achieved when A₀=a logic 0, and data is entered in the register during a write, WR,operation. Then, once the Address byte is written into the MultiplexRegister 54, Master Mux 24 will start transmitting the SYNC character orbyte (00010110) on the serial bus 16. The SYNC byte is derived from aSYNC Register 56. Synchronization of the SYNC byte and the subsequentdata bytes transmitted by Master Mux 24 is effected by a conventionalsynchronization circuit 58 receiving a synchronization control signalSYNC, from counter logic 60 which responds to the 25 KHz Mux CLK forproviding the appropriate phasing to the SYNC signal. Once the SYNC Byteis finished being transmitted on data line 16, the Address byte storedin register 54 will be converted from parallel to serial by register 62and is then automatically appended to the SYNC byte on the serial busvia synchronizing circuitry 58. An interrupt, INT*, is then generated.

The interrupt signal INT* is requesting that the processor 22 stop itsexisting action and service the MUX 24. A TINTRQ signal from the counterlogic 60 is applied to Interrupt Decision logic 64 and being extendedtherefrom as an interrupt request signal to the Interrupt Register andLogic 52 for extension as signal INT to the processor 22. Sometime afteran interrupt is requested by the Master Mux 24, the processor 22 willsend out an "interrupt acknowledge" (MI* and IORQ*). During this time,the Interrupt Logic 52 of Master Mux 24 will determine the highestpriority device which is requesting an interrupt. This is simply thedevice with its interrupt enable input, IEI, at a logic 1 and itsinterrupt enable output, IEO, at a logic 0. It will be noted in FIG. 3Bthat the IEI input of Master Mux 24 is tied to +5 volt and its IEOoutput is tied to the IEI input of the Monitor Mux 24'. The InterruptLogic 52 is such that when IEI is a logic 1, no other devices of higherpriority are being serviced by an interrupt service routine from theprocessor 22. Additionally, the IEO signal from the Interrupt Logic 52will be a logic 1 only if the IEI input to that Mux is a logic 1 and theprocessor 22 is not servicing an interrupt from that multiplexer. Thus,when the processor 22 is servicing an interrupt from that multiplexer,its IEO signal is a logic 0 and will serve to place the IEI inputs ofother multiplexers to which it is connected to a logic 0, thereby makingthem subservient in the priority of interrupt servicing. To ensure thatthe interrupt priority daisy chain is stabilized, Multiplex devices areinhibited from changing their interrupt request status when Ml* isactive (0). The highest priority device places the contents of itsInterrupt Vector Address Register 50 onto the data bus 40 during"interrupt acknowledge". After an interrupt by the Master Multiplex isacknowledged, that multiplexer is "under service". The IEO of thisdevice will remain low until a return from interrupt instruction(RETI=ED_(H) 4D_(H)) is executed while the IEI of the device is alogic 1. If an interrupt request is not acknowledged, IEO will be forcedhigh for one Ml* cycle after the multiplex 24 decodes the Op code"ED_(H) ". This action guarantees that the two byte RET 1 instruction isdecoded by the proper Mux device.

When the processor 22 receives the interrupt, it has one byte time (0.32ms) in which to write the Command byte into the Multiplex Register 54.If this time has elaspsed before data is written into the register 54,the data on the serial bus 16 following the Address byte will benonvalid. As was the case with the Address byte, the Command byte willbe transferred from register 54 through parallel to serial converterregister 62 for output to the serial data line 16 via the datamultiplexing sync circuit 58, the CRC data Mux gate 66 and thetransmitter circuitry 68.

As the serial Address and Command data bytes are being read fromregister 62 onto the serial data line 16, they are also being providedon the line designated "SDATA#2" to an input of an EXCLUSIVE OR gate 70having its output connected to an input stage of CRC calculator register72. The other input to the EXCLUSIVE OR gate 70 is provided by aselected one of the output stages of the CRC register 72 to perform theCRC calculation function in accordance with the description provided inthe aforementioned application Ser. No. 469,591. Immediately followingtransmission of the Command byte, the CRC byte will have been generatedin register 72 and is serially read therefrom and through the CRC DataMux circuit 66 which has been enabled by a CRC word select signal CWSfrom the counter logic 60.

Upon completing the transmit mode, the Master Mux 24 automaticallyswitches into the receiver mode. In the receiver mode, the Remux Addressbyte will be the first character to be received, after which aninterrupt will be generated to signal the processor 22 that it is timeto read that byte. Once again the processor has 0.32 ms in which to readthe byte, after which the byte is no longer valid. The data beingreceived from line 16 by Master Mux 24 first enters through receivebuffering circuitry 74 and is gated through logic circuitry 76 to theserial to parallel register 62. A transmit/receive control signalTNRECNTL applied to gate circuit 76 provides for the received data, RDATA, to be passed to serial to parallel register 62 and also providesthat that data be extended to an input of the EXCLUSIVE OR 70 for use inthe calculation of a CRC byte during the receive mode.

In the receive mode, the second byte will be the Remux response byte andit will be received in the same manner as the remote address byte. Ittoo will signal the processor 22 with an interrupt indicating that validdata can then be read. The final byte received by Master Mux 24 will bethe Remux's CRC byte. This byte will be compared to the byte generatedby the CRC calculator 72 from the incoming data stream during thereceive mode. If the two bytes compare, bit number 0 in the Master Muxesstatus register 78 will be a 0, whereas if the two CRC bytes do notcompare bit number 0 in register 78 wll be a 1. Also, an interrupt isagain generated to signal the processor 22 that the status registershould be read. Reading the status register 78 serves to reset thatregister. In addition to the state of bit number 0 in status register 78serving to indicate the correctness or incorrectness of the CRC from theRemux, the state of bit number 1 will be indicative of operation in thetransmit or the receive mode, the state of bit number 2 will beindicative of the correctness or incorrectness of the CRC comparison ifthe device is operation in the monitor mode as will be discussedhereinafter. The state of bit number 3 signifies whether or not theMaster Mux and Remux addresses compare when the device is operating inthe monitor mode as will be described hereinafter and the state of bitnumber 4 is used to signify when a transaction has been completed.

When the status register 78 has been read and reset, the Master Mux 24will switch back to the transmit mode and will send ones onto the serialbus 16; thus waiting for a write, WR*, from the processor 22 to initiateanother transaction.

Brief reference to the timing diagram provided by FIGS. 5A-L furthercorrelates the timing of the above-described functions with the variousparts of a complete transaction performed by Master Mux 24. It will benoticed in 5K that a single interrupt is generated during the transmitmode, whereas three separate interrupts are generated during the receivemode. It will also be noticed that the control signal DLOAD in 5E forloading parallel data into the parallel to serial register 62 occurs atthe beginning of the address, command and CRC bytes respectively duringthe transmission mode, whereas that function to convert from serial toparallel occurs at the end of the address, response and CRC bytes duringthe receive mode. The determination of whether or not the CRC bytereceived by Master Mux 24 was correct is determined by the signal CRCOK? appearing in 5I at the end of the CRC byte appearing at the end ofthe transaction. A Master Address/Receive Address comparison signalM/RCOMP? occurs at the end of the receipt of the Remux address duringthe receive mode of the transaction; however, this function is performedonly by Monitor Mux 24' as will be discussed hereinafter.

Referring now to the operation of a multiplex controller in its monitormode, as represented by Monitor Mux 24', the monitor mode may beattained either upon power-up initialization or by writing a 0 into bitnumbers 0 and 3 of the control register 46. In this mode the multiplexerfunctions only as a receiver or listener, and interrupts to theprocessor 22 are never generated. In such monitor mode the INT* istri-state. After establishing a particular Mux as a monitor 24', anoutput line designated bus listen control BUSLCNTL from the bit number 0position of control register 46 serves to control Master/Monitor selectcircuitry 80 for generating a control or gating signal MONLY which is a1 when the device is to operate as a Master Mux and is a 0 when it is tooperate as a monitor. The MONLY control signal is extended to thoseportions of the Master/Monitor Mux circuitry which are to providedifferent modes of operation depending upon whether the chip isconfigured as a master or as a monitor. When configured as a Monitor Mux24' , the device operates only as a receiver and therefore is in the"sync search" state awaiting receipt of a Sync byte at the "downstream"end or terminal of the serial data line 16. When the Sync byte isreceived, it is recognized by Sync Detect Logic 63 and the Monitor Muxis initialized for enabling the following three bytes on the serial busto be acknowledged. The first byte received after the Sync byte is theRemux address transmitted by the Master Mux. This byte is stored in thedata buffer, Mux Data Register 54, for comparison later with the addressbyte sent by the responding Remux 14. Further, that first byte is gatedthrough the CRC data Mux 76 and EXCLUSIVE OR gate 70 to the CRCcalculator 72. The next byte received is the command from the Master Mux24 and it also is gated through CRC data Mux 76 and EXCLUSIVE OR 70 intothe CRC calculator 72 for determining a CRC byte value at the monitor.The third byte received by monitor 24' will be the Master's CRC. ThisCRC byte is also conducted through the CRC data Mux 76 and the EXCLUSIVEOR 70 to the CRC checker 72 to determine whether or not the CRC bytescompare. If the results are the same, bit number 2 in the statusregister 78 will be set to a 0 and if not, a 1 will be placed in bitnumber 2 of that register. After this operation, the CRC calculator 72is automatically cleared.

The fourth byte received by Monitor Mux 24' is the address byte sent bya responding Remux 14. This byte is compared to the Address bytepreviously sent by Master Mux 24 and presently stored in Mux dataregister 54. The comparison of the two address bytes occurs in AddressCompare Logic 84 which is active only if the Mux chip is acting as amonitor 24'. If the two address bytes compare, bit number 3 of theStatus Register 78 will be set to 0 and if not, a 1 will be place in bitnumber 3 of that register. Byte 4 received by Monitor Mux 24' is alsopassed through CRC data Mux 76 and EXCLUSIVE OR gate 70 into the CRCcalculator 72 for use in determining a CRC byte. The fifth byte receivedby Monitor Mux 24' is the Response byte issued by the responding Remux14. This response byte is directed through CRC data Mux circuit 76 andEXCLUSIVE OR gate 70 to the CRC calculator for determination of the CRCerror byte.

The sixth byte received by Monitor Mux 24' is the CRC byte sent by theresponding Remux 14. This byte is also directed through CRC data Mux 76and the EXCLUSIVE OR gate 70 for comparative combination with the CRCbyte then stored in register 72. In the event the CRC byte transmittedby the Remux compares with that calculated in the Monitor Mux 14, a 0will be set in the bit number 0 position of the status register 78, butif the bytes do not compare, a 1 will be placed in the bit number 0position of the status register. Upon completion of this transaction,the SYNC search mode will be restored; thus enabling a new operation tobegin.

Although the Monitor Mux 24' is not capable of generating an interruptto the processor 22, the processor will instead interrogate the MonitorMux 24' following each transaction to ascertain whether or not theaddress and CRC checks performed by the monitor indicate accuracy,integrity and consistency in the transmission between the Master Mux 24and Remux 14. In the event errors are reflected by one or more of thesechecks, the processor 22 is capable of taking various forms ofcorrective action.

Prior to a further discussion of the centralized control of themultiplexing system 10 by Mux computer 12, the circuitry of arepresentative Remux 14 will be described in somewhat greater detailwith reference to FIGS. 1 and 6. Firstly referring to FIG. 6, arepresentative Remux 14 is illustrated in functional block diagrammaticform. The Remuxes 14 are provided by LSI gate array logic configured toprovide the requiste functions discussed herein. Each Remux 14 isconnected with the Mux bus loop 15 via parallel "T" connections with therespective four wires 16-19 of that bus. The provision of +5 volts andground is not shown but is implied. The bidirectional data lineconnecting Remux 14 with the Mux data line is designated 16' and theline connecting the Remux with the MUXCLK line 17 is designated 17'.Appropriate circuitry 120 is located in line 17' for buffering thereceived MUXCLK. Similarly, in line 16' there is provided Receivebuffering circuitry 121 and Transmit buffering circuitry 122 connectedin complementary relationship. The incoming data on line 16' and theMUXCLK on line 17' are supplied as inputs to an eight bitserial-to-parallel and parallel-to-serial shift register 125 which isresponsible for providing the appropriate conversions of data from oneform to the other in response to an appropriate control signal CRTL. Asused in the description of FIG. 6, the control signals CRTL are providedby control logic 128 and may provide a variety of control functions. Thefolloing discussion is intended to reveal the characteristics of thosevarious control functions. Each of the Remuxes 14 on the Mux bus 15awaits receipt of a Sync byte from the Master Mux 24 as an indication ofa transaction to follow which may or may not be addressed to aparticular Remux 14. The Synch byte is received on Mux data line 16' andentered in the register 125 where it is parallel connected via bus 130to control logic 128 capable of recognizing the Synch byte pattern andproviding a resulting control signal. Accordingly each Remux 14 is thensynchronized to receive the next byte in the transaction from the MasterMux 24, that being an Address byte containing the address of aparticular one of the Remuxes 14.

Each of the Remuxes 14 will have a different preassigned address storedin address latch 132. The address latch 132 and the preprogramming of anaddress thereinto will be described later in greater detail. The addressbyte received on the serial data line 16' is compared by the ControlLogic 128 with the Remux Address previously stored in address latch 132.The address for only one of the Remuxes 14 should compare andaccordingly, the other Remuxes are effectively dormant for the remainderof that particular transaction. However, for the Remux 14 actuallyaddressed, the bidirectional transfer of data will continue.

During the receipt of the address byte from Master Mux 24, that bytewill be directed through CRC data Mux circuitry 134 and an EXCLUSIVE ORgate 136 into CRC calculating and checking circuitry 138 in the mannerpreviously described for the Master and Monitor Muxes 24, 24'. Upondetermination that a Remux 14 is the particular one being addressed, thenext incoming byte, the Command byte, will also be directed throughcircuit 134 and the EXCLUSIVE OR gate 136 into the CRC circuitry 138.Serial to parallel circuitry 125 applies the eight bits in the commandbyte to the A side or the B side latched outputs for additional decodingprior to latching outputs 140A, 140B respectively. The Command byte datamay also be applied to the Control Logic 128 via bus 134 for effectingcertain control actions, such as Read A Side or B Side switches; ReadEssential switches; and Diagnostic Byte. As was earlier mentioned, eachRemux 14 is divided into halves, one half being designated the A sideand the other half being designed the B side. Although a particularRemux 14 has a singular distinct address relative to all of the otherRemuxes 14, the distinction between A side and B side within aparticular Remux is determined by whether the least significant bit inthe address is even or odd. An even LSB designates the A side of a Remux14 and an odd LSB designates the B side of the Remux. Internal logicassociated with Control Logic 128 is capable of determining whether theLSB is even or odd and thus the address latch 132 is only required toprovide the seven more significant bits of the address. When both theaddress byte and command byte have been received by Remux 14, aresulting CRC value has then been determined in the calculator 138. Thenext byte to be received is a CRC byte transmitted by Master Mux 24 andit is introduced to the checking circuitry 138 via CRC data Mux 134 andthe EXCLUSIVE OR gate 136 to determine whether the two CRC bytescompare. Unless they do compare, the Command byte previously receivedwill not be executed. However, assuming the CRC bytes do compare, one ofthe following commands will be executed: (1) Send all A-side debouncedswitch inputs; (2) Send all B-side debounced switch inputs; (3) Senddiagnostic byte for even address; (4) Send diagnostic byte for oddaddress; (5) Send undebounced B-side switch inputs; (6) Write a 1 to aspecific output line for the A side or for the B side; (7) Write a 0 toa specific output line for the A side or for the B side; (8) Write a 1to all eight output lines for the A side or for the B side; (9) Write a0 to all output lines for the A side or for the B side. The execution ofthese commands requires converting the serial Command byte to paralleldata applied via bus 130 to the Control Logic 128 so as to set certainA- or B-side latched outputs to a particular state or to condition thecertain gating 142 such that it can transmit input switch states fromeither the A-side debounced switch register 144A, the B-side debouncedswitch register 144B or the essential switch register 146 to theparallel-to-serial register 125 via parallel bus 148. Further, a controlsignal CTRL from the Control Logic 128 may condition the register 125 tosend a particular diagnostic byte.

The Reply half of the handshake transaction between Master Mux 24 andRemux 14 begins with the Address byte identifying the particular Remux14 and further, the A or B side thereof, which is responding. Thisaddress is derived from the address latch 132 and Control Logic 128 andis parallel loaded into register 125 for serial shifting out via serialline 150 which is extended through the transmit buffer 122 to the Muxdata line 16'. During this reply transmission of the Remuxes address, itis also entered into the CRC calculator 138 via CRC data Mux 134 and theOR gate 136 in a manner before described. The response byte next sent byRemux 14 may be coded in a variety of ways to indicate the responsetaken by the Remux to the previously received command byte. Forinstance, if either the A side or the B side debounced switch inputs144A, 144B or the undebounced B side switch inputs from register 146were requested, each bit of the eight bit Response byte will signify andan input state of a respective one of the eight input switches. Each ofthe responses to each of the remaining commands discussed above is auniquely coded eight bit byte indicative of a positive acknowledgementthat a commanded output action has been effected. In this latter regard,the output latches are monitored to determine that they were set to thestate commanded and a coded Response byte indicating compliance is thenformulated and transmitted. As with the Address byte, the Response byteis formed in shift register 125 and is serially shifted out on line 150for transmission on line 16'. Similarly, the response byte is entered inCRC calculator 138 for determination of a Reply CRC byte. Uponformulation of that CRC byte, it is parallel shifted via bus 152 gatinglogic 142 and bus 148 into register 125 for serial transmission via line150 to the Mux data line 16'. Upon completion of transmission of the CRCbyte from Remux 14, the transaction is completed and the Remux returnsto a state in which it awaits another sync byte and more specificallyits address from the Master Mux 24. The Remux 14 is of course providedwith power-on resetting circuitry (not shown) for initializing thecircuitry each time it is powered up.

A facet of the invention is illustrated in FIGS. 6, 7, and 8 concerningthe provision of switch input signals to a Remux 14. It will be noted inFIG. 6 that MUXCLK received on line 17' is extended, after frequencydivision of 128 by divider 137 via internal line 17" to the A sidedebounced switch register 144A and the B side debounced switch register144B. Each of the registers 144A and 144B includes eight synchronouslyclocked debouncer circuits adapted for receiving inputs fromsingle-throw switches and providing debounced signals to the remainingcircuitry of the Remux for transmission during a response byte. FIG. 7further illustrates one of the essential switch inputs 146₁ contained inthe essential switch register 146. The individual debouncer circuits forthe A and B side registers 144A and 144B are designate 144A₁ and 144B₁,respectively. Because the debouncing circuitry 144B₁ associated witheach of the B side switches is identical with that of the A side switchdebouncers 144A₁, only the latter will be described in detail. In thosesituations employing a single throw switch for input of a control signalto a digital logic system and particularly a logic system which issynchronously clocked, it is often advantageous to "debounce" theresulting signal in order to prevent an inaccurate indication ofmultiple switch opening and closings upon a single switch actuation. Inthe illustrated embodiment, many of the input switches 160 are of thesingle pole, single throw momentary contact type, and the potentialproblem of switch bounce may be significant. Accordingly, the debouncedcircuitry A₁ is particularly suitable for detecting actuation of asingle or multiple pole single throw switch and providing a clean,debounced synchronized signal for use by the internal circuitry of Remux14 or other similar circuits.

One terminal 161 of single-throw switch 160 is connected to ground(logic 0) and the other terminal 162 is connected through input buffer163 to an input port of EXCLUSIVE OR 164. A positive voltage of 5 voltsis applied to the input terminal 162 via resistor 166 to hold the inputside of switch 160 at a logic 1 during its normally-open configuration.It will be appreciated, however, that the debouncer circuitry 144A₁ isequally suitable with a single-throw switch of normally-closedconfiguration. The other input to EXCLUSIVE OR gate 164 is provided vialine 168 from the Q output of a latch, such as D-flip-flop 170. Theoutput of EXCLUSIVE OR 164 is extended via line 171 to respective inputson NAND gate 172 and AND gate 174. For synchronization with theremaining internal circuitry of Remux 14, the MUXCLK signal supplied tothe Remux on line 17' is processed via control logic 128 to provide anACLK signal and an ACLK* signal operative in 180° phase relationshipduring interrogation of the A side inputs, and a BCLK and BCLK* signalsoperative in 180° relationship during interrogation of the B sideinputs. The ACLK signal is applied via line 175 to another input of NANDgate 172. A third input to NAND gate 172 is provided via line 176 fromthe Q output of latch 17. The other input to AND gate 174 is providedfrom the Q* output of a latch, such as D-flip-flop 178, via line 179.The output of AND gate 174 is extended via line 180 to the D-input oflatch 178. The ACLK* signal is extended via line 181 to the CLK input oflatch 178. The output of NAND gate 172 is extended via line 184 to theCLK input of latch 170, the Q* output of that latch being connected vialine 185 to its D-input. Finally, an optional preset signal may beapplied via line 186 to the Asychonrous Clear (AC) inputs of the latches170 and 178.

Reference is now made to the operation of a debouncer circuit 144A₁,with particular reference to the timing diagram of FIG. 8 whichillustrates the signal waveforms at various locations throughout thecircuit. The timing waveforms in FIG. 8 are designated according to thenumbering in FIG. 7 of the signal line or component on which or at whichthey appear. The input from switch 160 appearing at terminal 162 isillustrated as normally being high until switch actuation, whereupon itgoes low; however, due to bounce there are several alternations betweenthe high and low logic states for an interval designated T_(B). Theinput signal 162 is applied to EXCLUSIVE OR gate 164 as is the resultingdebounced output signal 168. As long as these signals are equal, i.e.,both 1's or both 0's, the output 171 from EXCLUSIVE OR 164 is the logic0 and no changes occur in the latches 178 and 170. If the two inputssignals to EXCLUSIVE OR gate 164 differ, the output 171 from EXCLUSIVEOR gate 164 is a logic 1. If the signal on line 171 is a logic 1,latches 178 and 170 may change state if the duration of the differingsignals at the input of EXCLUSIVE OR gate 164 exceeds a capture timewindow equal T_(DB) plus T_(A) for the circuit. The expression T_(DB) ofthe capture time window represents the interval or period betweensuccessive down-steps in the ACLK signal. The expression T_(A) for thecapture time window represents that variable interval between theinitial down-step in the switch input signal 162 occurring upon initialswitch actuation and the next down-step in the ACLK signal (or up-stepin ACLK*). The ACLK signal is a debouncer clock derived as mentionedfrom the transmitted MUXCLK such that its falling edge, or down-step, istime-synchronous with the system MUXCLK edge at which input data isdesired to transition. When the switch input signal 162 does change as aresult of actuation of switch 160, it occurs asynchronously with respectto ACLK, the interval of that asynchronism being designated T_(A) andbeing equal to or greater than 0 but being equal to or less than theinterval T_(DB).

If the signal 171 appearing at the output of the EXCLUSIVE OR gate 164remains a logic 1 for a second ACLK period following switch actuation,the Q output 168 of latch 170 will toggle due to a clock signal 184 thenappearing on its CLK input. This will result in the output 171 of theEXCLUSIVE OR gate 164 then returning to 0 since the Q output 168 oflatch 170 is then the same logic state as the switch input signal 162.In any event, the Q output 176 of latch 178 will toggle to a 0 whenclocked by the second ACLK* pulse following switch actuation, and inconjunction with AND gate 174, will set up to detect the next change inthe switch input signal 162. Typically that next change will be anopening of the switch 160 to return signal 162 to a logic 1; however,the interval until that event will typically be determined by theconstruction of the switch and the duration of its actuation. Forinstance, although the trace 162 of switch input illustrates the switchopening after only several ACLK periods, it will be appreciated that itmay be longer if the switch is of the momentary type and the usercontinues to depress the switch. Still further, the switch may remainactuated when the user removes finger pressure, in which case theinterval of actuation would continue until the switch is manuallyopened.

Thus it will be seen that changes of state of the switch input signal162 that do not exceed the capture time (T_(DB) plus T_(A)), asresulting from bounce, will not result in the output 168 from latch 170being toggled. Accordingly, a false input signal is not provided as aninput to the remaining circuitry of the Remux 14. This is due to theoutput signal 171 from EXCLUSIVE OR gate 164 returning to the logic 0state prior to the occurrence of a second ACLK pulse following anyparticular switch actuation causing a change of state.

It will thus be appreciated that the output 168 of debouncer circuit144A₁ is a clean, debounced input signal which may then be applied tologic such as AND gate 190 such that it may be gated to the remainingrelevant portions of the Remux 14 by a gating signal 192 which isprovided when an A-side input switch is being interrogated. Asmentioned, the debouncing circuitry 144B₁ for each of the B side switchinputs is identical to that just described.

In accordance with another facet of the invention to be describedhereinafter in greater detail, some of the input switches are associatedwith so-called "essential" functions. Examples of such essentialfunctions include exterior lighting such as head and tail lights, hazardflashers, keyless entry system, and various others. Because variousportions of the Mux system 10 may be placed in a "sleep" mode inaccordance with a later described aspect of the invention, it may not bepossible to provide clock signals such as ACLK, BCLK, etc., which arerequired for the proper debounce operation and latching of input signalsusing only the debouncing circuitry of 144A₁, 144B₁. Therefore, thoseinput switches identified as being "essential" will also be capable ofproviding their input via the essential switch register generallydesignated 146 in FIG. 6, an individual such essential switch registerbeing illustrated in greater detail in FIG. 7 and designated 146₁. Forconvenience, all essential switches in the present system have beenassociated with the B side input. Thus, it will be seen that the switchinput signal 162 extending from a B side input switch in FIG. 7 isextended via line 194 to an essential switch register 146₁, and morespecifically to one input of a pair of NAND gates 195, 196 which arecross-coupled in a conventional manner to provide a conventionallatching circuit. The other input to this latching register 146₁ is aresetting signal, RSTESW* provided via line 197 for resetting the latchfollowing its interrogation. The output signal from the essential switchlatch 146₁ appears on signal line 198 where it is held following switchactuation and awaiting interrogation.

A further aspect of the invention resides in the provision of atechnique and circuitry for the automatic self-addressing of integrateddigital devices. More specifically, the self-addressing circuitry isparticularly applicable to large scale integrated circuits where thepackage pins available for I/O are at a premium. In the embodiment ofmultiplex system 10 under consideration, such a situation exists inconnection with the LSI gate array circuits which make up each of theRemuxes 14. Accordingly, provision is made in accordance with theinvention for establishing the address in the address latch 132 of theRemux 14 illustrated in FIG. 6 using the same I/O ports as are used forthe output signals from the A side latched outputs 140A. Although inthat illustration only seven address entries are made in latch 132 andeight pins are provided for the output of signals from outputs 140A, itwill be appreciated that the total number of pins, i.e., eight, areavailable to the output circuitry and to the address input circuitry.Referring to FIG. 9 there is illustrated in generalized form, circuitryfor implementing a time-shared method of digital control output andvariable address input on the same actual signal pins of an integratedcircuit device, as for instance Remux 14. Assuming some number, N, ofI/O pins 200 are provided for time-shared output of data and input ofaddress, it is also necessary to provide one additional I/O pin 201 toserve as an Address Common bus as will be described. Conventionalexternal power-on-reset circuitry comprised of resistor 202 andcapacitor 204 generate a power-on reset signal POR which is a normalinput signal to Remux 14 provided via a normal existing pin 206 for use,following squaring and inversion by inverter 208 as a reset signal 210extended to various other portions of the integrated circuitry of Remux14. Additionally, signal 210 is applied to the enable gate input of atri-state device 212 having its input connected to ground (logic 0), andhaving its output connected via line 214 to the Address Common I/O pin201. The inverted state of reset signal 210 is obtained on line 216 atthe output of inverter 218.

The group of A side latched outputs 140A each include eight latchesindividually designated 220, and the address latch group 132 includesseven address latches individually designated 222. Accordingly in FIG. 9each D-flip-flop latch 230 for the A side outputs is shown grouped witha respective D-flip-flop address bit latch 232 within a dotted blockdesignated 220, 222. Each function block 220, 222 also includes arespective transceiver comprised of a tri-stated inverting transmitteroutput circuit 240 and a nontri-stated inverting receiver input circuit242 of conventional design. The Q* output of output latch 230 isconnected as the input to the transmitter circuit 240, the output ofwhich is connected to a respective one of the I/O pins 200 for extensionvia a respective lead to respective external load circuitry (not shown).Similarly, the input to receiver circuit 242 is connected to the samerespective I/O pin 200, its inverted output being connected to theD-input of address latch 232. The RESET* signal appearing on line 216 isconnected to the power-on reset, PRST, input of each of the outputlatches 230 and to the clock, CK, input of each of the address bitlatches 232. The RESET* signal on line 216 is also connected to theenabling gate of each tri-state device 240. The D-input of each outputlatch 230 is supplied with a respective gating signal via a respectivelead 250 from the interior logic of Remux 14. A clock signal is suppliedto the clock input, CK, of each output latch 230 via respective clockingleads 252 from the interior circuitry of Remux 14.

Finally, a preselected number of diodes 260 are connected withpreselected ones of the I/O pins 200, but not necessarily all, inaccordance with the particular address to be established and entered inthe latches 232 of the integrated circuitry of Remux 14. Morespecifically, each diode to be used in establishing the addressconfiguration will have its anode connected to a respective one of theI/O pins 200 and will have its cathode connected to a Common line 262which extends from the Address Common pin out 201 to the cathodes ofeach of the other diodes 260 to be used in the address configuration. Aseparate resistor 264 is connected at one end to a respective one of theoutput pins 200 and at the other end to +5 volts (logic 1) for normallyholding the I/O pin 200 at a logic 1 voltage level. Similarly, thislogic 1 voltage will be applied to the anodes of those diodes 260 usedin the selected address configuration.

Having thus described the circuitry for the line 210 time-shared methodof digital control output/variable address input on the same I/O pins200, it is appropriate to discuss the operation of that circuitry withadditional reference to the timing diagrams of FIG. 10. When power isfirst applied, capacitor 204 is charged through a resistor 202 andprovides a RESET and RESET* pulse to the circuitry via lines 210 and 216respectively. Tri-state circuit 212 is of known design and normallyprovides a high impedance on the address common pin 201 and line 262;however, when the RESET signal on line 210 is a 1, the tri-state device212 is also active and applies a logic 0 level on the Address Common I/Opin 201 and line 262. This will clamp any of the I/O pins 200 associatedwith address diodes 260 to the logic 0 level. Indeed, when the diodes260 are clamped to the logic 0 level, their anodes, the I/O pins 200 andthe lower end of resistors 264 will similarly be at a logic 0 level, theresistors typically being about 2.7 ohm and drawing 1.6 ma in thiscondition.

At the same time that the Address Common line 262 is actively low (0),reset inverter 218 is also low on its output line 216. The signal online 216-is then effective to place the tri-state devices 240 in thehigh impedance or Z state, permitting the logic states appearing at therespective ones of the I/O pins 200 to be input through the respectivereceiver devices 242 and in addition initializes the output latch 230.For those I/O pins 200 to which diodes 260 are not connected, therespective pull-up resistors 264 will maintain the pin at a logic 1level such that a 1 is applied to the receiver circuit 242. The receivercircuits 242 invert their respective inputs such that the D-input of arespective address latch 232 will receive a logic 1 if a logic 0 appearson the respective input pin 200 and will receive a logic 0 if a logic 1appears at that pin.

When RESET signal 210 is a 0, the RESET* signal 216 will apply a risingedge to the clock, CK, inputs of each of the address latch 232 andresult in these states latching the address bits received via receivercircuits 242. At the same time, tri-state circuit 212 goes to the highimpedance Z state and the RESET* signal on line 216 places thetri-stated transmitter devices 240 in the active condition, therebyreturning the I/O pins 200 to their normal role as device output pins.The unidirectional characteristic of the signal diodes 260 used in theselected address configuration serves an isolation or steering functionwhich prevents logic states on any of the I/O pins 200 which happen tohave addressing diodes 260 installed thereto from affecting each othervia the Address Common line 262.

The effect of this time shared use of the pins 200 for data output andfor address input is that all Remuxes 14 in multiplex system 10automatically program their address and do not require anyinitialization before they can be used. Moreover, from a device designviewpoint a total of N-1 pins are saved for an N-bit addressingrequirement. For example, if a 256 address architecture is assumed,eight address bits would be required for each device. This wouldnormally require eight dedicated package pins to implement per device orin the extreme alternative, 256 different device types which areidentical except for their address option which is fixed duringintegrated circuit manufacture. On the other hand, with theabove-described circuitry, eight existing output pins would also be usedfor self-addressing during initial power-up or power-on, and one AddressCommon pin (201) would have to be added to the package, resulting in anet saving in device pins of N-1=7 pins.

Having completed a consideration of the circuitry of Remux 14, it isappropriate to further consider the interaction of processor 22, MasterMux 24, Monitor Mux 24' and, via serial bus 15, the Remuxes 14. Becausethe system 10 includes a pair of selectable, redundant multiplexcontrollers 24, 24' under the direction of a processor 22 in its Muxcomputer, the system is provided with enhanced integrity andoperability. Firstly, the control program of the processor 22 isestablished such that it configures the Master and Monitor status of theMuxes 24, 24' to best ensure MUX DATA and MUX CLK transmission to andfrom the intended Remux 14. To this end one of the redundant Muxcontrollers is designated a Master and the other is designated aMonitor; however, their relative functions and designations may bereversed in the event the configuring program deems it desirable toeffect the communications objective of the system. Stated yet anotherway, in the event a line break occurs in Mux data line 16 or MUX CLKline 17, somewhere in the external ring or loop bus 15, the Mux computer12 is capable of recognizing this situation and reconfiguring orreversing the functional roles of the Master and Monitor Muxes 24, 24'.Referring to FIG. 11, the decisional process of processor 22 inconfiguring the Muxes 24 and 24' as Master and Monitor or Monitor andMaster, respectively, is depicted in flow diagram form. Referring toFIG. 12, the decisional and control process of processor 22 indetermining whether or not a transmission using a particular Mux as aMaster is "good" or "bad" is depicted in flow diagram form.

Referring first to the flow diagram of FIG. 11, the initial conditionsfor entering the "configuring" routine include either the power-upinitialization step 300 which occurs when power is first applied to thetotal system or a request for configuration provided as a result of step400 in the flow diagram of FIG. 12. In either event, step 302 obtainsthe address of the first Remux to be addressed in a predeterminedsequence of Remux addressing. In step 304 the control registers of theMuxes 24, 24' are set such that a preselected one, i.e., Mux 24, is aMaster and the other, i.e., 24' is the Monitor. Absent a need toreconfigure, i.e., due to multiplex line degradation, this relationshipwill continue to be maintained. Step 306 then effects transmission ofdata to and from the addressed Remux via the selected Master Mux 24.Decision block 308 broadly represents one or more tests for determiningthe integrity and success of the communication transaction. Assumingthat integrity is established, a flag 310 is set in the processor'smemory which indicates that transmissions addressed to that particularRemux should continue to use multiplexer 24 configured as a Master toeffect those transactions. On the other hand, if the integritydetermination step 308 results in a negative decision, the effort toaccurately complete the transaction to the addressed Remux via theinitially selected Master Mux 24 will be repeated some number of times,N, as represented by block 312, and decision block 314 monitors whetheror not the repeated attempts are successful. If an attempt is successful(Yes), the program routine returns to the affirmative output of block308. On the other hand, if after N tries (i.e., eight) the transactionintegrity is not established, the control program of the processor willreverse the relative roles of the Master and Monitor multiplexers suchthat the multiplexer 24 becomes the Monitor and the multiplexer 24'becomes the Master, as represented by block 316. This reversal offunctions is effected by reversing the logic states of the relevant bitsent to the relevant stage of the control registers of the respectiveMuxes. Then, as represented by block 318, the processor program will tryas many as N times to direct communications to and from the Remuxes 14through the former Monitor, now Master, Mux 24'. Decision block 320determines whether or not this effort through the reconfiguredarrangement of the Muxes is successful and assuming it is, theconclusion of block 322 is that a line break or the like has preventedthe Mux computer from addressing the particular Remux through originalMaster 24 and must now address that Remux using Mux 24' as the newMaster. Block 322 will set flags in memory so as to indicate a linebreak and to thereafter direct communications with that particular Remuxthrough Mux 24' configured as a Master. If however, the decision ofblock 320 was negative, that particular Remux will thereafter beconfigured out of the system of possible Remux addresses, as representedby block 323.

Returning to the main trunk of the flow diagram, and specifically to theoutput of block 310 and the input of block 324, an effective paththrough either of the Muxes 24 or 24' to the Remuxes addressed up tothat point will have been established or else a particular address willhave been configured out of the system. Block 324 asks whether all validRemux addresses have been configured and if not, an instruction isissued by block 326 to get the next Remux address and the routine thenreturns to the entry point to block 304. When all Remuxes have beenconfigured, the configuring routine is stopped.

Referring to the flow diagram of FIG. 12, there is illustrated thedecisional routine associated with "good" and "bad" communicationtransactions between the Mux computer 12 and the various Remuxes 14. Atthe entry 350 to this routine, a transaction between the Master Mux anda Remux has occurred, and the Address and Command sent by the Master andthe Address and Response received by the Master from a Remux are inmemory 25. Step 350 provides for reading the Status Registers in boththe Master and the Monitor Muxes 24, 24'. Based on that information,numerous decisions can then be made.

First, a check is made at block 352 to see if the "transaction done"flag for the Master has been set. If the transaction does not appear tobe done, the routine branches to an arm having block 356 which requiresthat the decision logic "wait one transaction period", a block 358 whichincrements a "resend" count, a block 360 for determining if the "resend"count is yet a maximum, and a block 362 for resending the message ortransaction if the "resend" count is not a maximum. If the "resend"count is a maximum, the routine jumps to the "configure" entry point 400into the configure routine illustrated in FIG. 11.

Assuming at block 352 that the transaction had been completed as isnormally the case, block 354 determines if the CRC at the Master fromthe Remux is valid. If that CRC is not good, as by failure of CRC's tocompare, the routine branches to a decision chain which first determinesat block 364 if the Address received by the Master and conveyed toprocessor 22 is hexadecimal FF, i.e., all 1's. Such a condition is aninvalid address and usually exists only if the MUX DATA line 16 is openand thus pulled to a constant logic 1 level. If the Address is not FF,the "resend" count is incremented at 358 and the message is resent at362 unless the resend count is a maximum. If the Address is FF, aMonitor status bit is checked at 366 to determine if the first half(i.e., Master transmit) CRC is bad. If it is not bad, the "messageresend" routine 358-362 and 400 is pursued; otherwise a Monitor statusbit is checked at block 368 to determine whether the second half (i.e.,Remux reply) CRC is bad. If it is not, the "message resend" routine358-362 and 400 is pursued; otherwise a Monitor status bit is checked atblock 370 to determine if the addresses sent by the Master and by theRemux fail to compare. If those addresses do compare, the "messageresend" routine 358-362 and 400 is pursued; otherwise a check is made atblock 372 to determine whether the "resend" count is some value X. Ifthis "resend" count is X, the routine jumps to the entry point 400 ofthe configure routine of FIG. 11.

Returning to block 354 in the main trunk of the "good-bad" transactionroutine, if the CRC from the Remux at the Master is good, a comparisonis made of the Address sent by the Master and the Address received bythe Master from a Remux. This comparison is made in the softwareassociated with processor 22, and is represented by blocks 374 and 376.If the Addresses do not compare, the "message resend" routine of 358-362and 400 is pursued; however, an optional "fast refresh" may first beprovided. The "fast refresh" serves to accelerate a "refresh routine"(not shown) for reinstating the Remux outputs. Assuming the Addresses docompare, the Response received at the Master and conveyed to processor22 is inspected at block 380 to determine if it is a hexadecimal FF. Ifit is FF, the "message resend" routine of 358-362 and 400 is pursued;otherwise the routine proceeds to decision block 382 to check whetherthe "line break" flag has been set in block 322 of the "configure"routine. If the "line break" flag has not been set, the routine moves toblock 384 to see if the Monitor's "transaction done" status bit has beenset. If it has been set, the next transaction sequence may proceed, asrepresented by block 386. It will be understood that the check for theMonitor's "transaction done" status might have occurred earlier in thedecision chain. Similarly, if the "line break" flag of 322 has been set,thus indicating that the Master and Monitor have been configured tocompensate for that situation, the next transaction sequence mayproceed. If the Monitor's "transaction done" flag is not set, theroutine branches through a "1 transaction period wait" at block 388 andthen to the "message resend" routine of 358-362 and 400.

The aforedescribed "good-bad transaction" routine serves to incorporatea number of integrity checks and provides for several attempts atresending a transaction before postulating that a line break exists andthat reconfiguration of the Master and Monitor Muxes may be required.

Reference is now made to an aspect of the invention illustrated ingreatest detail in FIGS. 1, 2, 3A and 3B in which provision is made foroperating the multiplex system 10, and particularly a portion thereofrepresented by the dotted block 500 in FIG. 2, in a low power mode forvarious intervals. Due to the limited capacity of the system's powersource which typically is an automobile 12 volt storage battery,provision is made for extending the life of that power source byminimizing the power consumed by the multiplex system 10 as a whole, andparticularly that portion of the multiplex computer 12 designated 500 inFIG. 2 which includes the microprocessor 22, the system clock source 28,the ROM 27 and the decode control 34, and thus being all of thecircuitry in FIG. 3A.

Prior to considering a functional description of the low power operationof the Mux system 10 via a so-called "sleep" mode, that circuitryexternal to the microprocessor 22 which is utilized in this process willbe discussed. Referring specifically to FIG. 3B, all of the circuitrydepicted therein is constantly connected to a +5 volt supply voltagederived from an automobile storage battery or the like. This circuitryis utilized in the control of the microprocessor 22 and in providingvarious timing functions utilized to implement the low power "sleep"mode and to thereafter "reawaken" the system.

The 50 KHz oscillator 30 is of conventional RC design and provides a 50KHz signal to the clock input CK1 of an eight stage counter 32. Thestages of the counter are connected so as to provide at its first Q_(A)output the 25 KHz square wave signal which is extended to the Master andMonitor Muxes 24, 24' as the MUXCLK. The final Q_(D) output of counter32 is connected to the CK1 clock input of counter 33 to continue theripple-down count provided by counter 32. It will be important to note,however, that the CL2 reset input for the second half of counter 32 andthe CL1 reset input to the first half of counter 33 serve to reset thoseportions of the counters 32 and 33 and thereby change their count cycleswhen reset pulses are provided. Reset pulses are under control ofprocessor 22 and usually follow NMI* pulses.

The Q_(A) output of the first stage of counter 33 is extended via line510 to one input of an AND gate 512, the output of which is extended vialine 514 to the CL2 reset input to the second half of counter 33. TheQ_(C) output of the third stage of counter 33 is extended via line 515to one of the inputs of NAND gate 516, the output of which is extendedto the D-input of D-flip-flop 518. The Q_(D) output of the fourth stageof counter 33 is extended via line 519 to one input of AND gate 520 andto the CK2 clock input of the second half of that counter. The Q_(D)output of the eighth stage of counter 33 is extended via line 522 to theinput of an inverter 523 and also provides the reset signal, RESET. ThatRESET signal is extended through inverter 524 in FIG. 3A to provide theRESET* input to microprocessor 22 and to also provide signal PUR₂applied to the MR* inputs of Master and Monitor Muxes 24, 24'.

The signal appearing on line 522 comprises one of the inputs to powercontrol circuitry generally included within dotted enclosure 35. Powercontrol circuitry 35 initiates and terminates the low power or "sleep"mode of operation. In addition to inverter 523, the power control logic35 includes a D-type flip-flop latch 526, an AND gate 528 and finally agated power switch such as MOSFET 530.

The AND gate 516, D-flip-flop 518, D-flip-flop 532 and NAND gate 534provide logic which, in conjunction with the counters 32 and 33,provides interrupt signals at 20 ms intervals as will be hereinafterdiscussed, and generally corresponds to pulse generator 36 in FIG. 2.Further, circuitry comprised of D-flip-flops 536, 538 and 540, as wellas AND gate 542, serve to reset counters 32, 33.

Under normal operation, the D-flip-flop 518 wll provide a nonmaskableinterrupt signal, NMI* for controlling operation of the program forprocessor 22. The NMI* signal is first provided 20 ms after the systemis first powered and a power-up reset pulse PUR1 has occurred.Thereafter, NMI* interrupt signals will be generated at 20 ms intervalsduring normal powered operation of the microprocessor 22. The NMI*signal is that transition occuring at the Q* output of flip-flop 518when the first memory request signal MREQ* is applied to the clock CKinput of the flip-flop following its D-input going to a logic 1 at 20 msintervals. That enabling of the D-input of flip-flop 518 at 20 msintervals is determined by line 515 from counter 33 and is conditionedupon the system being in its "on" or "awake" mode, as represented by thesignal appearing on line 550 applied to the other input of AND gate 516.The MREQ* signal is also extended to the CK input of D-flip-flop 532which has its D-input connected to the Q output of flip-flop 518. The Q*output of flip-flop 532 is provided as one input to NAND gate 534, theother input to which is provided by the Q output of flip-flop 518.Accordingly, a resetting signal coinciding with the NMI* pulse isprovided on line 552 at the output of NAND gate 534 at 20 ms intervalsand continues until the next MREQ* signal. The reset signal appearing online 552 is extended to the CL* input of D-flip-flop 536 which reenablesthe timer reset logic 538, 540 and 542.

Flip-flops 538 and 540 are initially cleared by a power-up reset PUR1upon initial application of power. Further, the Q* output of flip-flop536 is set to a logic 1 by the signal appearing on line 552 followingeach NMI* signal. This action sets flip-flops 538 and 540 such thattheir Q* outputs connected to the input of NAND gate 542 are both set to0 and 1 respectively when a 50 KHz clock is applied to their respectiveclock CK inputs. Accordingly, the output of AND gate 542 would then be 0and would not effect a resetting of timers 32 and 33. However, if themicroprocessor 22 is operating normally, after responding to a NMI*pulse it will generate a timer reset signal, TIMERST which is extendedto the clock CK input of flip-flop 536 for toggling first that flip-flopand then the two subsequent flip-flops 538 and 540 such that the outputof AND gate 542 goes to a logic 1. Such a logic 1 at the output of ANDgate 542 effects a resetting of the last four stages of counter 32 andthe first four stages of counter 33. Under normal operation, the TIMERSTresetting signal is provided at approximately 20 ms intervals andaccordingly, the counter 33 is reset before it proceeds to the nextcount following its 20 ms output from the third stage on line 515.

In the event the microprocessor 22 is operating incorrectly, isunpowered or becomes "lost" and fails to generate the TIMERST signal tothe counter resetting circuitry, the counters 32 and 33 will no longerbe reset and will continue their normal ripple-down count. Under suchcircumstances the counter 33 will continue to count beyond the 20 msinverval at which it would normally be reset and will reach a time of 40ms which is reflected by a logic 1 appearing on line 519 applied to theinput of NAND gate 520. If at that time the other input to NAND gate 520is a logic 1, as will be the case during normal powered operation, theNAND gate will output a logic 1 signal which is extended via line 556 tothe PR* input of latch 526 such that its Q output is set to a logic 1.Significantly, the power MOSFET 530 is "on" and provides +5 V_(SW) whenits gate input is a logic 0, and is off when its gate input is alogic 1. The Q output of flip-flop 526 is extended via line 558 to thecontrol gate of FET 530 such that when it goes to a logic 1, the FET isswitched off and +5 V_(SW) is disconnected from the circuitry 500. Whenthat +5 V_(SW) supply is disconnected from the microprocessor 22, thesystem enters a "power-down" or "sleep" mode.

Because the counter 33 is continuing to count down beyond the 40 ms timeat which it placed the microprocessor in the power-down or "sleep" mode,it may then provide a reenabling or "wake-up" signal at some later time.Specifically, when the interval since the last NMI* interrupt reachesabout 654 ms, the final stage of the timer 33 will go to a logic 1 whichis extended via line 522 to provide the aforementioned reset pulse RESETand to provide a wake-up signal to the input of inverter 523 in thepower control circuit 35. Inverter 523 will then provide a 0 at the CL*reset input to latch 526, thereby resetting its Q output to a logic 0which turns FET switch 530 "on" and reapplies power to themicroprocessor 22 and other circuitry 500. In this manner the logicrepeatedly tries to get the microprocessor 22 back into an operationalmode after a "watchdog" event by first powering down and then repoweringand, at the time of repowering, by extending a reset signal RESET to theprocessor to initialize it at each attempted restart. When and if theprocessor 22 is effectively restarted, it will resume generating TIMERSTpulses to effect resetting of timer counters 32, 33 for inhibitingsubsequent "watchdog power-down".

When the Q* output of power control latch 526 goes to a logic 1 at thetime of "wake-up", that 1 is extended via line 550 to an input of ANDgate 512. Following a short delay, i.e., 5 ms, the first stage ofcounter 33 will send a pulse via line 510 to the other input of AND gate512, resulting in a reset pulse being extended via line 514 to reset thesecond half of counter 33.

The power control switch 35 has been discussed in association with theinitiation of power-down in response to the watchdog circuitry timingout because no TIMERST resetting pulse occurred. However, that powerswitch 35 may also be used to power-down the microprocessor 22 and placeit in a "sleep" mode in response to a control signal provided by theprocessor. Specifically, if it is desired to enter a "sleep" mode theprocessor 22 may generate a "power-down" or "power-off" strobe signal,PWRSTB, derived via the timer decode logic 34C. The PWRSTB signal isextended to the clock CK input of the power control latch 526. Becausethe D-input to latch 526 is held at a logic 1, the application of thePWRSTB signal will result in the Q output of that register being set toa logic 1, thereby turning off the power FET 530. In this way theprocessor 22 can place itself and the other circuitry within block 500in the "sleep" mode such that all communications on the Mux Data line 16are stopped and the MUXCLK signal on line 17 is turned off.

The timing of the PWRSTB signal which initiates the "sleep" mode ispre-established to occur shortly after the issuance of a TIMERST pulsesuch that little or no time has elapsed since the timers 32 and 33 werereset. Accordingly, those timers 32, 33 will begin their timing atsubstantially the same time as the "sleep" mode is initiated by PWRSTB.When 654 ms have elapsed and the line 522 from timer 33 goes to a logic1, the power control register 526 will reverse states and the FET powerswitch 530 will again be turned on to "wake-up" or "power-up" processor22 and the other circuitry within block 500.

The power control switch 35 additionally provides, via the output of ANDgate 528 and the inverter 566, a Chip Select signal CS* which isextended to the CS* inputs of Master and Monitor Muxes 24, 24'. When theCS* signal is in its active low state while power is being supplied tothe microprocessor, it enables the Mux devices to accept control signalsfrom processor 22. On the other hand, when the CS* signal goes highduring a "power-down" interval or mode, the Muxes 24, 24' are preventedfrom accepting any signals from processor 22 and will place the MUXCLKsignal on line 17 at a constant high level for its duration.

Further, the power switch circuitry 35, via the output from AND gate 528and the NAND gate 568, will serve to inhibit the enabling of RAM 26 byRAM enable signals RAMENB applied to the other input of the NAND gate568 so long as the system is in the "power-down" or "sleep mode". Thisprovision is desirable inasmuch as the RAM 26 is not to be enabledduring the "power-down" mode and there is a chance that the RAM decodecircuitry 34B might inadvertently provide an enabling signal RAMENB ator during the "power-down" mode.

The operation of the Mux computer 22 and Remuxes 14 will now beconsidered in accordance with the functional flow diagram of FIG. 13which illustrates placement of the system in the low power of "sleep"mode and the subsequent return to a "power-up" status. This routine iscontrolled principally by the programming of the processor 22, whichprogram may be readily implemented by one skilled in the art in view ofthe description and flow diagrams. One portion of the routine does relyupon the hardware of timer 16 and the power control switch 35, as hasbeen and will be explained.

The various "essential" body electrical functions previously mentionedare those which must function independently of the vehicles' ignitionswitch position. For example, lights (exterior, headlamps, hazard, etc.)must be operable at all times and are thus an "essential" function. Onthe other hand, windshield wipers only operate when the ignition switchis "on" in the "RUN" or "ACCESSORY" position and are thus a"nonessential" function. The multiplex system 10 is powered directlyfrom the vehicles' battery rather than through the ignition switchbecause of these essential functions. However, the "sleep" mode providedherein serves to automatically reduce the power drain on the battery bythe multiplex system during periods of inactivity, such as when thevehicle is parked. The "sleep" mode and the "essential" switch inputs146 have been configured such that the multiplex system resonds toessential inputs within a reasonable time even though the system mayhave been in a "sleep" mode.

Referring to FIG. 13, the programmed processor 22 periodically looks atthe ignition switch, as represented by flow diagram block 600. Theignition switch (or switches, if more than single "on" and "off") isconnected to a B-side debounced input 144B₁ of a Remux 14, and thus alsois connected to an essential switch register 146₁. Decision block 602determines whether or not the ignition system is "off" and if it is not,the routine goes immediately to block 630 which tells the system toremain "on" or "powered-up" and to service the operator's commands. Ifthe ignition system is "off", the routine moves to step 604 which beginsthe "sleep" decision process.

The first determination of the "sleep" decision process, as representedby block 606, is whether the exterior light system or hazard lightsystem (or other "essential" function), is "on". If one is "on", theroutine goes to block 634 to remain "on", but if none are "on", a 10second monitoring time period is provided during which the power ismaintained, as represented by block 608. The 10 second monitoring periodis measured by the program software. During the 10 second monitoringperiod the system inputs continue to be scanned, as seen in block 610,and decision block 612 determines whether or not any "essential"function or ignition switch has been activated during the 10 secondmonitoring interval. If so, the system continues to poll switches andservice driver's commands, as represented at block 630, and the loopreturns to block 600. If a switch has not been activated during the 10seconds, the decision is made to enter the "sleep" mode at step 616.

The "sleep" mode sequence first provides, at block 618, for all theRemuxes 14 to be placed in their low power dissipation mode by settingtheir output latches 140A and 140B so as to deactivate the loads and bygating "off" the MUXCLK. Deactivating the loads puts the externalelectronic drives in the lowest power configuration. Gating "off" theMUXCLK puts the Muxes and Remuxes in the lowest power configuration dueto the fact that CMOS power consumption is directly related to thenumber of gate transitions in the device. Next, as represented by block620, a power-off strobe pulse PWRSTB is generated by the software forextension to the power switch latch 526. Block 622 indicates that thePWRSTB pulse and latch 526 operate to swtich "off" the +5 V_(SW) powerfor the Mux computer circuitry in dotted box 500 in FIG. 2. The powerfor the circuitry in box 500 is timed to remain "off" for 654 ms, withthe dotted line path to block 624 representing that this timing is ahardware function and that power is switched back on at the end of thetiming interval.

When power is restored to the full system at the end of the 654 mstiming interval as represented by block 626, the MUXCLK is restored tothe Remux devices and their "essential" switch registers 146 areinterrogated. Next, at decision block 628, the routine determineswhether or not any "essential" switch activity occurred during theimmediately preceding 654 ms "sleep" interval. If such activity did notoccur, the routine returns to block 620 to reenter the "sleep" mode. If,however, "essential" switch activity did occur, the routine proceeds toblock 630 where it continues to poll switches and service drivercommands. The routine then restarts the "low-power" decision at block600. It may be noted that no response action is taken based only on aninput from an "essential" register 146₁. Instead, the system seeksfurther confirmation from the corresponding B-side debounced switchwhile next "powered up". The power required by the multiplex system 10during the "sleep" mode is less than 10% of that required duringpower-up, and the average power consumption will be a function of theduration of the respective "sleep" intervals. Longer "sleep" intervalswill decrease power consumption, but must not be so long as tounreasonably delay response to an input from an essential switch."Sleep" intervals in the range of 250-750 ms are realistic, and asteady-state drain of less than 10 milliamperes on the vehicles' 12 voltbattery has been attained during the "sleep" mode.

Although this invention has been shown and described with respect todetailed embodiments thereof, it will be understood by those skilled inthe art that various changes in form and detail thereof may be madewithout departing from the spirit and scope of the claimed invention.

We claim:
 1. A serial data communications system for providingbidirectional serial data communication between a central controlstation and one or more remote stations, comprising:central stationcontrol means including a central controller and at least two multiplexcontrollers; serial signal bus means having terminals at two oppositeends, said serial bus means being arranged as a loop having one said endterminal connected to one of said multiplex controllers and having theother said end terminal connected to the other of said multiplexcontrollers; a remote multiplex controller at each of said remotestations, each said remote multiplex controller including input/output(I/O) signal ports for input and output respectively of signalsrespectively from and to remote input devices and remote load devices,each said remote controller being parallel "T" connected to said serialbus means intermediate said end terminals thereof and having arespective address and being responsive to a serial communicationcontaining said respective address, from said central control stationfor providing a response to said communication, a said responseincluding input of signals from said remote input devices or output ofsignals to said remote load devices and a data reply to said serial busmeans via said connection therewith; and said central controller beingoperatively connected for communication with each of said multiplexcontrollers, each of said multiplex controllers being selectablyresponsive to instruction by the central controller for effecting serialbidirectional data communication transactions as a master with addressedones of said remote controllers, said central controller includingsignal processing means for selecting one of said multiplex controllersto be said master in accordance with a preselected bias and forselecting the other of said multiplex controllers to be said master whendata communication of acceptable integrity with an addressed one of saidremote controllers by said one multiplex controller is unattainable. 2.The communications system of claim 1 including means for comparing aportion of said data reply from a said remote controller with a portionof the data transmission to an addressed said remote controller fromsaid master controller to thereby provide a signal indicative of whetheror nor communication of acceptable integrity is attainable with saidaddressed remote controllers.
 3. The communications system of claim 2wherein each of said central station multiplex controllers isstructurally substantially the same as the other and includes arespective control register for effecting operation of the respectivemultiplex controller as either a master or a monitor, a said monitormultiplex controller being able only to receive data on said serial busmeans from said master multiplex controller and from respective saidremote controllers replying to said master multiplex controller, saidcentral controller signal processing means being operative to set aparticular logic state in a particular stage of said control register ofa selected one of said multiplex controllers to effect operation of saidselected multiplex controller as a master.
 4. The communications systemof claim 3 wherein only one of said two multiplex controllers may be amaster at any particular time, the other being a monitor.
 5. Thecommunications system of claim 4 wherein said master multiplexcontroller and said monitor multiplex controller are separatelyaddressable by said central controller signal processing means forobtaining status data therefrom, both said master and said monitormultiplex controllers including logic means for determining whether ornot an attempted communication from said master multiplex controller toan addressed said remote controller is of acceptable integrity andproviding status data indicating that result, said signal processingmeans querying said master and monitor multiplex controllers for saidstatus data and being responsive to said status data indicating apredetermined condition of unacceptable integrity of said attemptedcommunication for reconfiguring said multiplex controllers such thatsaid functions of said master and said monitor are reversed.
 6. Thecommunications system of claim 5 wherein each said remote multiplexcontroller includes means for generating an error check byte as aportion of its reply communication predicated on a preceding portion ofsaid reply communication, said master and said monitor multiplexcontrollers each including the same means as a said remote multiplexcontroller for generating an error check byte as a function of theirreceipt of said portion of said reply communication, means in each ofsaid master and said monitor multiplex controllers for providingrespective status data indicative of whether or not said remotecontroller's error check byte compares with that determined locally,said central controller signal processing means being responsive tocertain indications for effecting said reconfiguring of said master andmonitor multiplex controllers for communications with said addressedremote multiplex controller, said certain indications includingindications by both said master and said monitor multiplex controllersthat said error check bytes failed to compare for a particular addressedremote multiplex controller.
 7. The communications system of claim 5wherein said control system is initialized upon initial powering up ofthe system, and thereafter said signal processing means is operativeunder program control for addressing said remote controllers in apredetermined polling sequence, the initial said polling of said remotecontrollers serving to determine the selection thereafter of aparticular multiplex controller for use as a said master to address aparticular said remote multiplexer, said multiplex controllerpreselected to be said master prior to said initial polling beingpreferred in said selection determination for each particular addressedremote multiplexer with which communication is satisfactory during saidinitial polling sequence, otherise the said multiplex controllerpreselected to act as monitor is selected to act instead as master forcommunication with said particular remote multiplexer.
 8. Thecommunications system of claim 7 wherein said signal processing means isoperative in response to failure to successfully communicate with aparticular said remote controller using both said multiplex controllersalternately as Masters to record said failure in program memory forthereafter excluding said particular remote controller from the pollingsequence.
 9. The communications system of claim 6 wherein said monitormultiplex controller includes means for comparing the address bytecontained in a communication sent by a master multiplex controller to aremote controller with the address byte contained in a respective datareply from a remote controller for providing respective status dataindicative of the result of said comparison, said central controllersignal processing means further requiring an indication by said monitorstatus data that said address bytes fail to compare before effectingsaid configuraton of said master and said monitor multiplex controllers.10. The communications system of claim 5 wherein said serial signal busmeans consists of four conductors including a first conductor for serialmultiplex data, a second conductor for multiplex clock signals, a thirdconductor for one logic power level, and a fourth conductor for anotherlogic power level.
 11. The communications system of claim 10 whereinsaid central control signal processing means comprises a microprocessorand each said multiplex controller is a large scale integrated circuit,and including parallel bus means extending between said processor andsaid multiplex controllers for providing address, data and controlcommunications therebetween.
 12. The control system of claim 11 whereinsaid multiplex controllers each include logic means for generating aninterrupt signal, only said master multiplex controller being enabled toissue a said interrupt signal, said interrupt signal being extended tosaid microprocessor for instructing said microprocessor to request datafrom said master multiplex controller as soon as possible.
 13. Thecommunications system of claim 1 wherein said signal processing meanseffects plural unsuccessful attempts at data communication between saidpreselected master multiplex controller and a particular said addressremote controller before making a decision that communication ofacceptable integrity using said preselected master multiplex controlleris unattainable.
 14. The communication system of claim 6 wherein saidmaster multiplex controller includes means for generatng an error checkbyte as a portion of its transmission predicated on a preceding portionof said transmission, said monitor multiplex controller includes meansfor generating an error check byte, as a function of its receipt viasaid serial bus means of said portion of said master's transmission onwhich said master's error check byte was generated, said monitorincludes means for receiving and comprising said error check bytereceived from said master with said error byte generated by said monitoras based on said master's transmission and providing status dataindicative of comparison or noncomparison, and wherein said certainindications causing said central controller signal processing means toreconfigure said master and monitor multiplex controllers furtherinclude a said indication of noncomparison of said error check bytesbased on said master's transmission.